Photostimulable luminescent (PSL) technology is used to make x-ray images without film. Such technology is generally disclosed in U.S. Pat. No. 3,859,527 and U.S. Pat. No. RE 31,847. Using this technique, a PSL substrate is used to acquire the x-ray image. The material of the substrate has the property of temporarily storing x-ray energy for later release when stimulated by high intensity visible light or infrared radiation. The material is first completely erased by flooding with visible light to return it to its ground state. Next, it is exposed to x-ray radiation which raises the material to its long term excited state in proportion to the two dimensional variation in the intensity of the x-rays. The image is then read out by scanning with a high intensity visible light beam and then measuring the photostimulated radiation of the PSL material.
PSL substrates are used to make x-ray images with the same x-ray generating equipment as used with conventional photographic film/intensifying screen combinations. In a conventional x-ray examination, the film and intensifying screen are mechanically supported and protected from room light by a cassette of metal or plastic during x-ray exposure. Later, the film is removed from the cassette and photographically developed to produce the x-ray image.
A PSL substrate also requires a cassette so that it can be used with ordinary x-ray equipment. In the past, cassettes similar to those used with x-ray film were modified for use with PSL substrates. After x-ray exposure, the substrates were removed manually and inserted into the scanner apparatus or a special mechanism was required to unload the substrate from the cassette. This technique is described in an article by Sonoda et al. in Radiology, Volume 48, pgs. 833-848 (1983). After scanning and erasing, the PSL substrate had to be manually or mechanically loaded back into the cassette. The process of removal of the substrate from the cassette, separate scanning, and reloading back into the cassette involved mechanical wear or damage to the substrate surface. Since the image resulting from the scanning process cannot contain substantial artifacts, even slight scratches in wearing of the surface are important and are the main factor in limiting the useful lifetime of the PSL substrates.
The physical dimensions of the cassette surrounding the PSL substrate are limited by the fact that the cassette must be physically similar to those used with film so that it will fit into the cassette holders built into ordinary x-ray generating and exposing equipment.
Mechanisms for automatically removing and loading PSL substrates from cassettes are difficult to manufacture and use for several reasons. First, the mechanism must be carefully designed so as not to damage the surface of the substrate. Second, since the cassettes come in several sizes, and since they have had to be separated from the substrate during scanning, the mechanism must provide for a separate path for each size so that the substrate and cassette can be reliably sorted and reunited after scanning and erasing. If a new size is introduced, the mechanism must undergo a substantial redesign. Third, since the cassette unloading and loading mechanisms are distinct from the scanner transport mechanisms they must be designed to work together without the possibility of damage to the substrate surface. This greatly complicates the overall design of the scanner.
The cassette must not interfere with the detection of the x-ray image. It should only minimally attenuate the x-rays transmitted through the patient or the object examined before they enter this PSL substrate. The cassette should also be designed so that it can absorb x-rays transmitted through the PSL substrate if scatter from surrounding structures is a problem. The cassette may conversely be designed to not attenuate these transmitted x-rays so that it may be used in x-ray systems where the transmitted x-rays are measured to automatically control the exposure.